I've never used AS3 my self but have heard only positive things about it, and Im sure you have the same experience in the past.
The only thing that comes to my mind is that AS3 might not be suitable for Fx cards ... but that can't be right ?
By the way, was the original compound/thermal pad on the core thick ?
I've never replaced anything on gfx cards but perhaps it needs to be a thicker layer then on a cpu ? *must be some guides around*
I was thinking about changing the heatsink on my old gf3 and replace it with some active cooling and add some ram sinks and push it up a bit more when I have new card in its place. Experiment alittle ...